Hi,
Andreas Kostyrka wrote:
> Well, how does pgcluster serialize statements?
AFAICT, per statement, ordering them by incoming time...
> And does it apply
> statements done on two connections concurrently?
Hm. I'm unclear what exactly you are asking.
I think I can give a better example, without the confusing now()
function, which is already nondeterministic by itself:
initialize a table with:
CREATE TABLE test (a int NOT NULL);
INSERT INTO test VALUES (1);
then concurrently run this insert on multiple nodes:
UPDATE test SET a = (SELECT max(a) + 1 FROM test);
Normally, in READ COMMITTED mode, the subselect might or might not yet
see the other's update. There's a race condition, which normally gets
solved with a FOR UPDATE.
In SERIALIZABLE mode, this gives you a serialization error.
I don't know how exactly PgCluster handles FOR UPDATE, but if you simply
replicate all statements and wait for every cluster node (as in
PgCluster's reliable mode, I guess), you will get exactly the same
behavior regarding this issue.
So, the above transaction should better be something like:
BEGIN;
x = (SELECT max(a) + 1 FROM test FOR UPDATE);
UPDATE test SET a = x;
COMMIT;
This works for both isolation levels and AFAIK with PgCluster as well.
Mr. Mitani, please correct me if I'm wrong.
Regards
Markus
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